This invention relates to a xenon-metal halide lamp having improved thermal balance characteristics associated therewith. More particularly, this invention relates to such a xenon-metal halide lamp as exhibits a specific lamp envelope shape that insures a balanced thermal distribution within the discharge chamber so as to result in a lamp capable of extended life and higher brightness.
Xenon metal halide lamps have been finding greater and greater use in the lighting field recently, particularly in the automotive lighting field or any other field where a high brightness light source with instant-on capabilities is required. One example of such a high brightness light source can be found in U.S. Pat. No. 5,239,230 by Mathews et al and assigned to the same assignee as the present invention and which is herein incorporated by reference. In this patent, a high brightness light source is disclosed having specific performance characteristics such as wall-loading, tensile strength of the lamp envelope material, convective stability and lamp operating voltage and mercury density; such characteristics being cooperatively balanced so as to achieve such high brightness with an arc discharge gap which is on the order of 4 millimeters or less in length, and operating at a fill density  greater than 50 mg/cc ( greater than 50 atmospheres). A central lighting system utilizing this high brightness light source is included in the commercial product offered by General Electric Company""s Lighting Business as the Light Engine(copyright) centralized lighting system.
Such a centralized lighting system offers many advantages to lighting designers including the obvious advantage of requiring less space for light fixture or delivery devices; that is, equipment or devices that are needed for mounting and reflecting, refracting or otherwise delivering the light output in the desired pattern. In an automotive application for instance, it is a great advantage to disposing the light source away from the front end of the vehicle so as to allow more freedom in aerodynamic body styling of such vehicles. Having achieved success in designing a high brightness light source that can be disposed in one location and have the light output efficiently transmitted to one or more remote locations, the lamp designer still has other challenges to optimizing the design of such a high brightness light source. For instance, it would be desirable to provide the above described light source in a configuration that achieved a longer life expectancy than is presently achievable in spite of the extremely high operating pressure of the fill gas that is necessary to provide both high brightness and instant light. For instance, it is known that because of the pressure and temperatures at which the above-described light source operates, it has been found that this light source has a life expectancy of approximately 2000-4000 hours whereas it would be desirable that such a lamp exhibit a life expectancy on the order of about 2-3 times such a level.
In discovering a means for extending the life of such a high brightness light source, it was first necessary to understand the mechanism by which the end of life lamp failure occurred. Through empirical measurements taken using the above-described commercially available Light Engine light source, it was determined that for a xenon-metal halide lamp operated in a vertical orientation and powered by a DC source, a strong convection cell is generated inside the arc chamber of the lamp thereby causing a higher temperature at the cathode (upper) end than at the anode (lower) end and limiting lamp life thereby. Accordingly, it was determined that in order to extend lamp life to the desired level of approximately 6000 hours, it was necessary to find some way to limit the temperature gradient between the anode end and the cathode end of a DC powered, vertically oriented high brightness light source.
One known way for limiting temperature rises in a lamp is by use of a heat sink device. One such heat sink arrangement for a metal halide light source can be found in U.S. Pat. No. 5,204,578 issued to Dever et al on Apr. 20, 1993 and assigned to the same assignee as the present invention. In this patent, it is disclosed that a metal strip or cylindrically shaped metal piece can be disposed in contact with the outer surface of the arc tube chamber so as to draw heat thereto and away from the ends of the arc tube at which the electrodes are disposed. Though effective in operation with a light source that can be mounted individually within a headlamp assembly for instance, such a heat sink arrangement for a centralized light source which must couple light as efficiently as possible to remote locations, is not practical because of the amount of light that is blocked by the externally disposed metal pieces. Accordingly, it would be advantageous if a means for substantially reducing the thermal gradient between the anode and cathode elements of a DC operated, vertically oriented high brightness centralized light source could be developed that did not block light output.
It is also known that for the thermal operating characteristics of a light source with an elongated vertical arc tube, the convective heat load at the upper end of said elongated vertical arc tube is proportional to the arc tube radius to the fourth power. This relationship is discussed by D. M. Cap in the paper xe2x80x9cGrashof Numbers and Swirling Arcsxe2x80x9d, Advanced Engineering #931, published Sep. 2, 1970. Though providing guidance relative to the property of convection velocity and thus heat loading, such an approach is not sufficient to attain a high brightness, short-arc discharge light source such as provided by the above-referenced Light Engine lighting system. For such a light source, one must consider maintaining the design features necessary to achieve the high brightness characteristics. From the above-referenced Mathews patent for the Light Engine light source, it is known that to achieve the desired level of brightness, certain design parameters must be simultaneously satisfied. For instance, to achieve a brightness level in excess of 50,000 lumens per square centimeter of arc gap unit area, the mercury density must be within a specific range of values, the arc gap must be less than approximately 4 millimeters, and the wall loading must be less than 25 watts per centimeter squared of arc tube surface area, and preferentially approximately 20 watts per centimeter with a tensile strength of a certain value to ensure the integrity of the arc tube. In order to meet these and other design requirements, a number of parameters must be balanced so that optimizing one or some of the parameters does not result in destabilizing the lamp or reducing the brightness output. Accordingly, it would be advantageous to design a high brightness light source with a unique envelope structure that would result in improved thermal operating properties for the light source without risking a loss in the amount of light output otherwise attainable.
One example of a light source having a non-ellipsoidally shaped arc chamber can be found in U.S. Pat. No. 4,594,529 issued to de Vrijer on Jun. 10, 1986. This patent discloses an elongated arc chamber but does not address the problems associated with lamp life related to heat load properties; the elongated arc is provided for the purpose of achieving a long arc discharge which is horizontally oriented and is utilized as a single direct source of light rather than a high brightness light source which is centrally located and remotely distributed.
Another problem associated with the operation of the high brightness light source at high pressure such that a significant thermal gradient exists between the cathode end (top) and the anode end (bottom) of the arc chamber was that, because of the higher operating temperatures at the top region, a pool of metal halide could not exist therein; the only metal halide pool available for use in the arc discharge came from the bottom region. Therefore, it would be advantageous if a high brightness light source could be developed that provided thermal operating conditions that allowed for the temperature at the inside top surface of the vertically disposed arc chamber to be comparable to the temperature at the inside of the arc chamber thereby allowing a larger area at which the metal halide pool could reside.
The present invention provides a high brightness, short arc gap light source having an extended life characteristic relative to other high intensity discharge lamps operating at high pressures and having high brightness light output capabilities. The inner dimensions of the lamp envelope are shaped so as to interrelate with one another and result in a reduction in the vertical temperature gradient along the inside surface of the arc chamber.
In accordance with the principles of the present invention, there is provided a high brightness light source comprising a lamp envelope having an arc chamber formed therein as well as a pair of electrode members which extend into the arc chamber and have a preselected spacing provided therebetween. Energizing means are connected to the electrode members so as to power the light source and result in the generation of an arc discharge within the arc chamber, the arc discharge having associated therewith, certain thermal operating properties. The light source is operated in a vertical orientation such that one of the electrodes, the cathode in the case of a DC operated light source, is disposed at the top region of the arc chamber. The arc chamber is constructed so that the inner diameter thereof is sufficiently small to control the overheating of the top of the arc chamber by limiting convective flow and is essentially uniform in dimension from top to bottom. By such shape and dimensional relationship, the thermal operating properties of this lamp are such that substantially equal operating temperatures are achieved at the inside top and inside bottom surfaces of the arc chamber in spite of the extremely high operating pressure of the fill gases. Moreover, by such an arc chamber configuration, the light source of the present invention operates such that the operating temperatures are even lower at the top region of the arc chamber than at the lower regions, allowing for additional wall coverage of the molten metal-halides at the top inside surface of the arc chamber. The highest inside surface temperatures are located at the same height as the arc gap, so that the quartz surface in that region remains clear of metal-halides, allowing maximum collection of the light emitted from the arc by the optical collection system of the light source.